Information Systems Security

Mahmoud Youssef, PhD

Professor Vijay Atluri

Based on notes by

The 3 Steps to Security Policy

– defines the requirements need to be implemented within the hardware and software external to the computer system including physical, personnel and procedural controls

– lays out broad goals without specifying how to achieve them Mechanism

– implements the requirements of the policy– one has to make sure that the mechanism performs intended

functions Assurance

– provides a measure of how well the mechanism meets the requirements of the policy

– low assurance mechanisms are easy to implement but also easy to subvert

– high assurance mechanisms are notoriously difficult to implement

Components of Security

Confidentiality Integrity

Availability

Confidentiality: Prevention of Unauthorized disclosure of informationIntegrity: Prevention of Unauthorized modification of information

Availability: Prevention of Improper denial of access to information

Network Security

Alice wants to send a private message to Bob over a public network

– What if someone intercepts and reads this message? (Confidentiality)

– What if someone intercepts and alters this message? (Integrity)

– What if someone pretending to be Alice forges a message and sends it to Bob? ( Authentication)

– What if Alice denies sending of the message? (Non-repudiation of origin, Digital Signature) page 52 of the text gives a distinction between the two

– What if Bob denies the receipt of the message? (Non-repudiation of the destination)

Cryptography A tool for confidentiality, integrity, authentication,

non-repudiation, and digital signatures Works for both passive and active attacks Cryptology

– cryptography: the science of encryption (the good guys)– cryptanalysis: analysis of cryptographic algorithms (the bad

guys) Cryptosystems

– Secret Key (also known as single key, symmetric key) existing for more than 1000 years

– Public Key (also known as two key, asymmetric key) since 1974 both secret key and public key systems are in use and competing with

each other

Secret Key Cryptosystem

EncryptionAlgorithm

DecryptionAlgorithm

Plain Text Plain TextCipher Text

A B

Key= K Key= K

Secure Channel

C = E(K,M)M = D(K,C)where K = keyE = Encryption AlgorithmD= Decryption AlgorithmM = Plaintext MessageC = Ciphertext Message

Insecure Channel

C = E(K,M) M M M = D(K,C)

Features of Secret Key Encryption

Uses:– Solves confidentiality and integrity problems– Can be used for Authentication– Can be used to securely store information on insecure

media– Integrity check

Disadvantages:– Key Distribution Problem: How to get the key to Alice

and Bob? and to others?– If everyone knows the Key, it is no longer a secret

Basic Encryption Techniques

Substitution – Simple Alphabetic Substitution

Huge key space: 26! (approximately 10^26) Trivially broken for known plaintext attack Easily broken for ciphertext only attack (for natural language

plaintext) Multiple encipherment does not help

Permutation Example: Caesar ciphers Key space: N ! for a block size of N Trivially broken for known plaintext attack Easily broken for ciphertext only attack (for natural language

plaintext)– Multiple encipherment does not help

Combinations and iterations of substitution and permutation

ABCDEFGHIJKL....FPAQFZYTLWXM..

1 2 3 4 3 1 4 2

Data Encryption Standard (DES) DES is a product cipher with 56 bit key and 64 bit block size for

plaintext and ciphertext Developed by IBM and adopted by NIST (1977) with NSA

approval for unclassified information efficient to implement in hardware, but relatively slow if

implemented in software E and D are public, but the design principles are classified Algorithm:

– initial permutation– the 56 bit key is used to generate sixteen 48-bit keys– 16 rounds of substitution and permutation are performed– swap left and right halves– Final permutation

the size of the key (56-bits) is one of the most controversial aspects of DES

How Secure is DES? has stood up remarkably well against 20 years of public cryptanalysis

– 1977: approved as a Federal standard with 5 year cycle of re-certification– 1987: reluctantly reapproved for 5 years– 1992: reaffirmed by NIST

DES known plaintext attack– 56-bit key can be broken in 2^55 = 3.6*10^6 trials

trials/second Time required

Time required

Time required

1 10^9 years 10^15 years 10^6 years10^3 10^6 years 10^12 years 10^3 years10^6 10^3 years 10^9 years 1 year10^9 1 year 10^6 years 10 hours

10^12 10 hours 10^3 years 40 seconds

56 bits 76 bits 46 bits (3.8*10^22 trials) (3.5*10^13 trials)

Remarks on Secret Key System

Distribution of the Key is a Problem Trillions of keys may be required because we

need at least n(n-1) different keys if we have n customers

Public Key Cryptosystem– solves the problem of key distribution provided a reliable

channel for communication of public keys can be implemented

Public Key Cryptosystem

security is based on infeasibility of computing B's private key, given the knowledge of

– B's public key– chosen plaintext– chosen ciphertext

EncryptionAlgorithm

DecryptionAlgorithm

Plaintext PlaintextCiphertext

A B

B's Public Key

Reliable Channel

B's Private Key

C = E(KE-B,,M)M = D(KD-B,C)where KE-B = Public (encryption) key of B, known to allKD-B = Private (decryption) key of B, known only to B E = Encryption AlgorithmD= Decryption AlgorithmM = Plaintext MessageC = Ciphertext Message

Digital Signatures in RSA

EncryptionAlgorithm

DecryptionAlgorithm

Plaintext PlaintextCiphertext

A B

B's Public Key

Reliable Channel

B's Private Key

DecryptionAlgorithm

EncryptionAlgorithm

Plaintext PlaintextCiphertext

A B

A's Private Key

Reliable Channel

A's Public Key

Compare with Encryption in RSA

E EPlain Text

D

Signatures and Encryption

D

SignedPlain Text

EncryptedSignedPlain Text

SignedPlain Text Plain Text

A's Private Key

B's Private Key

B's Public Key

A's Public Key

A B

we could do the encryption first followed by the signature. Signature first has the advantage that the signature can be verified by parties other than B

We could use DES for encryption